In an evaporator in which the liquid flows as a thin film on the heat exchange surface, the surface should be covered with liquid throughout in order to achieve an efficient transfer of heat. When a liquid suspension which contains solid ingredients is being evaporated, increasing concentrations of solids at the boundaries of the dry and liquid-covered areas of the heat exchange surface would additionally cause very rapid soiling of the heat exchange surface. An even distribution over the heat exchange surface of the liquid to be evaporated is thus an absolute prerequisite for efficient operation of the evaporator.
The keeping of the heat exchange surface covered with liquid can be promoted by increasing the liquid amount fed onto the surface. However, this increases the consumption of energy for pumping and increases the thickness of the flowing liquid film, thus lowering the heat transfer coefficient. For this reason, very narrow liquid conduits have generally been resorted to in liquid distributors in evaporators, but such conduits tend to become clogged when solids-containing liquid suspensions are being evaporated.
From Finnish lay-open print 86961 there is known a heat exchanger intended for the evaporation of liquid, the heat exchanger having adjacently positioned bags formed from plastic membrane, the membrane surfaces of the bags serving as heat exchange surfaces. The liquid to be evaporated is introduced onto the outer surfaces of the bags from conduits adjacently positioned in honeycombs at the upper end of the evaporator and distributed over the entire width of the bags. The heating medium used is vapor directed via the same honeycombs to the inside of the bags, the vapor being vapor produced during evaporation and compressed in a compressor.
The apparatus described in FI publication 86961 is intended for the distillation of seawater. In addition, the publication mentions as an intended use of the apparatus the concentration of solutions and suspensions, such as bleaching effluents. However, in this case the apparatus has the deficiency that the fiber material and other solids present in the suspension tend to clog the obliquely oriented narrow liquid conduits in the honeycombs.